1. Field of the Invention
The present invention relates to a linear drive mechanism using an electromechanical conversion element for moving mechanical components of cameras and other precision machines.
2. Description of the Related Art
When a drive pulse comprising a moderately rising part followed by a sharply falling part is applied to a piezoelectric element, a displacement extending in the thickness direction of the piezoelectric element is gently generated by the moderately rising part of the drive pulse, and a sharp compression displacement is generated by the sharply falling part of the drive pulse. Linear drive mechanisms are known which use this characteristic to generate a linear movement of a movable member friction-bonded to a drive member by applying a drive pulse of the aforesaid waveform to a piezoelectric element and repeatedly discharging at different speeds to produce vibration in the piezoelectric element in the thickness direction at different speeds, thereby reciprocally moving at different speeds a drive member fixedly attached to said piezoelectric element (e.g., Japanese Laid-Open Patent Application No. 6-123830).
FIG. 4 shows an example of the construction of a photographic lens drive mechanism of a camera using the aforesaid type of linear drive mechanism. In FIG. 4, reference number 51 refers to a lens barrel at the left end of which is fixedly mounted a holding frame 52 of a first lens L1, and at the right end of which is formed a holding frame 51a of a third lens L3. A holding frame 53 of a second lens L2 is disposed within lens barrel 51 so as to be movable in the optical axis direction. Reference number 54 refers to a drive shaft for driving lens holding frame 53 in the optical axis direction, said drive shaft 54 being supported by a first flange 51b of lens barrel 51 and a flange 52b of lens holding frame 52 so as to be movable in the optical axis direction, and one surface of a piezoelectric element 55 is fixedly adhered to one end of said drive shaft 54.
Piezoelectric element 55 is displaced in the thickness direction, thereby displacing drive shaft 54 in the axial direction; one end of piezoelectric element 55 is fixedly adhered to drive shaft 54, and the other end of piezoelectric element 55 is fixedly adhered to the second flange 51c of lens barrel 51.
The lens holding frame 53 supporting the second lens L2 is provided with a contact member 53b as a movable member extending in a downward direction, and drive shaft 54 passes through said contact member 53b. A notch 53c is formed on the bottom surface of the contact member 53b. The contact member 53b and drive shaft 54 are friction-bonded by a suitable friction force via pressure contact of a pressure spring 53d inserted between the top surface of contact member 53b and the notch groove 53c. FIG. 5 shows the construction of the contact portion between drive shaft 54 and contact member 53b; the view is a section view along the X--X line of FIG. 4.
Another notch (not shown in FIG. 4) is formed at the top of lens holding frame 53, and engages a guide shaft 59 so as to prevent rotation of lens holding frame 53. Reference number 60 refers to a mount for attaching the lens to a camera.
The control operation is described hereinafter. When it is necessary to move lens L2 in the arrow a direction, a drive pulse having a waveform comprising a moderately rising part followed by a sharply falling part is applied to piezoelectric element 55, as shown in FIG. 6.
The piezoelectric element 55 generates a moderate displacement extending in the thickness direction, and displaces the drive shaft 54 in the axial direction indicated by arrow a.
Thus, lens holding frame 53 can be moved in the arrow a direction because pressure spring 53c presses against drive shaft 54 so as to produce a frictional bond with contact member 53b of lens holding frame 53 and causing movement in the arrow a direction.
The sharply falling part of the drive pulse generates a rapid compression displacement in the thickness direction of piezoelectric element 55, such that drive shaft 54 is also displaced in an opposite axial direction to the direction of arrow a. At this time, lens holding frame 53 does not move because the contact member 53b of lens holding frame 53 pressed against drive shaft 54 by pressure spring 53c substantially stays at said position due to the inertial force produced by the friction force between said contact member 53b and drive shaft 54.
In this case "substantially" pertains to movement in the arrow a direction and the opposite direction producing a follow-up movement of slipping of the frictional bonded surfaces between drive shaft 54 and the contact member 53b of lens holding frame 53, and includes an overall movement in the arrow a direction caused by the difference in drive times.
The lens holding frame 53 can be consecutively moved in the arrow a direction by applying consecutive drive pulses of the aforesaid waveform to piezoelectric element 55.
When moving the lens holding frame 53 in the opposite direction to the arrow a direction, such movement can be accomplished by applying to piezoelectric element 55 a drive pulse having a waveform comprising a sharply rising part followed by a moderately falling part.
In the aforesaid construction, drive shaft 54 is displaced in a relatively narrow range following the telescopic displacement produced in piezoelectric element 55, and contact member 53b which is in a state of friction contact with drive shaft 54 slides so as to follow in a predetermined direction. When drive shaft 54 is relatively long, however, i.e., when the length of the drive shaft is such that there is a long distance from the end of the drive shaft in contact with the piezoelectric member to the part friction-bonded to the contact member, the drive shaft is not displaced in following the telescopic displacement of the piezoelectric element due to elastic deformation of the drive shaft, thereby producing a phenomenon wherein the contact member cannot move. Therefore, there are limitations on the length of the drive shaft which are inconvenient inasmuch as the driving range of the lens L2 cannot be increased.
The above described construction of a lens device is a generally used lens barrel construction wherein a first lens L1 and lens holding frame 52 shown in FIG. 4 are removable and lens L2 is fixed or capable of extension from the camera, but such a construction is disadvantageous when a linear drive mechanism is applied inasmuch as lens L2 can be retracted into the lens barrel but the drive shaft protrudes a constant length regardless of the position of lens L2, such that the length (thickness) of the fixed lens barrel including the drive shaft or the camera in the optical axis direction cannot be made more compact.